A polypropylene resin in-mold foaming molded product prepared from polypropylene resin pre-foamed particles is characterized by its freedom of shape, lightness, and heat insulation efficiency, which are advantages of an in-mold foaming molded product. Further, the polypropylene resin in-mold foaming molded product is superior to other similar in-mold foaming molded products. That is, the polypropylene resin in-mold foaming molded product is superior in (i) chemical resistance, heat resistance, and distortion recovery rate after compression to an in-mold foaming molded product prepared from polystyrene resin pre-foamed particles and (ii) dimensional accuracy, heat resistance, and compression strength to an in-mold foaming molded product prepared from polyethylene resin pre-foamed particles. Because of these characteristics, the polypropylene resin in-mold foaming molded product prepared from the polypropylene resin pre-foamed particles has been used for various applications for example for an automobile interior part and a core material for an automobile bumper, as well as a heat insulating material and a buffering packaging material.
The polypropylene resin in-mold foaming molded product prepared from the polypropylene resin pre-foamed particles is required to have an important property such as dimensional stability at high temperatures.
The polypropylene resin in-mold foaming molded product is often used in a high-temperature atmosphere. For example, an automobile interior part may be locally subjected to a temperature of almost 100° C. when used during the summer inside a car with its windows closed. Further, a heat insulating material, a buffering packaging material, a returnable box etc. are also often used in a high-temperature environment.
In view of this, the polypropylene resin in-mold foaming molded product is required to express excellent dimensional stability even in a high-temperature atmosphere. To this end, generally, a polypropylene resin in-mold foaming molded product prepared from polypropylene resin having a high melting point is employed.
However, use of such polypropylene resin having a high melting point causes the following problem. That is, the high melting point necessitates a high molding heating steam pressure when pre-foamed particles made from the polypropylene resin are subjected to in-mold foaming molding. This leads to an increase in molding process cost.
Therefore, a polypropylene resin in-mold foaming molded product that (i) can be molded by in-mold foaming molding at a low molding heating steam pressure and (ii) is excellent in dimensional stability at high temperatures has been desired.
Meanwhile, in recent years, a technique of catalysis of polymerization of polypropylene resin has been widely studied, and various studies are to be carried out to improve properties of polypropylene resin. Under the circumstances, there has been obtained polypropylene resin having a composition distribution sharper than that of conventional polypropylene resin (for example, Patent Literature 1). The polypropylene resin having the sharp composition distribution is achieved by controlling the composition distribution by using a so-called metallocene catalyst.
Further, there have been known pre-foamed particles made from polypropylene resin using a metallocene catalyst (for example, Patent Literatures 2 through 7).